not sure exactly but if you do write one general PS, don't use a line where you can easily tack in the name of the school your applying to.

I've met with enough admissions people and school reps to know that this is very see-through and usually gets your app dumped.

I think, from talking to a good number of people, that if you write one general PS you should be fine. However if the school does have an option for an additional essay on a topic they give, you could consider doing that in addition to your PS.